1. Field of the Invention
The present invention relates to contact-type image sensor modules used by image reading apparatuses in identifying valuable securities and paper bills, and in particular, it relates to such modules and apparatuses that enable prevention of defocusing phenomena caused by chromatic aberrations, when reading by polychromatic light sources using infrared light sources, and enable high accuracy reading.
2. Description of the Related Art
A conventional contact-type image sensor module is disclosed in Japanese Patent Publication No. 3,011,845, wherein, in order to prevent defocusing phenomena when reading documents, a focus-position is determined by providing a protrusion for a conveyed document and by contacting the document against the top of the protrusion. Moreover, by coating infrared light shielding paint on the protrusion only visible light is allowed to shine in. FIG. 14 is a cross sectional configuration diagram illustrating the structure of a contact-type image sensor module, disclosed in the publication No. 3,011,845. In FIG. 14, numeral 21 denotes a light source, numeral 22 denotes a lens, numeral 23 denotes a sensor IC, numeral 24 denotes a sensor substrate, numeral 251 denotes a glass plate, numeral 26 denotes a housing, numeral 27 denotes a connector, numeral 28 denotes a document, numeral 29 denotes a conveying platen, and numeral 210 denotes a protrusion.
Next, the operation will be explained. The light emitted from the light source 21 passes through the glass plate 251 and the protrusion 210, and arrives at the portion of the document 28 having text and character. The light is absorbed by the black portions that are the text, while the light is reflected approximately 100% in the white portions that have the background color of the document. The reflected light passes through the glass plate 251 and the protrusion 210, is focused by the lens 22, and is received by light receiving portion of the sensor IC 23 on the sensor substrate 24. The sensor IC 23 comprises a plurality of light-receiving parts where the light incident on each light-receiving part is photoelectrically converted, and a driver portion from which the output power is extracted. The received light is converted into an electrical signal in the driver portion, and outputted as image information through the connector 27. Text, etc. written on the document 28 is consecutively read out line by line, while the document 28 is conveyed by the rotation of the platen 29.
When using a monochrome light source or a white light source (e.g. a cold-cathode tube) which includes a plurality of light components, because the light source itself is monochrome and is driven singly, problems related to chromatic aberrations with respect to light sources have been ignored, since reading monochrome documents has been the main object. However, in situations where color documents are read and the light source is in a plurality of colors and driven independently, the light reflected by the document focuses on the light-receiving part of the sensor IC at different conjugate lengths, due to the chromatic aberrations in the rod lens, etc. In particular, when using an infrared (IR) light source, the conjugate length is extremely longer than the red (R), green (G) and blue (B) light which is alternative light sources. The conjugate length between the surface of the document and the light-receiving part of the contact-type image sensor modules is generally determined by the R, G and B (the three fundamental colors for color reading) light, therefore when reading the document using IR light, the resolution may deteriorate. Accordingly, there has been a problem of being unable to read all of document colors in high resolution.
Moreover, light shielding technology represented by infrared filters has been used in order to block out infrared light itself. Consequently, when using an infrared light source, blocking out the infrared light has been originally unnecessary on principle.